A bioregion is a geographical area defined not by administrative boundaries, but by distinct characteristics such as plant and animal species, ecological systems, soils and landforms, human settlements, and topographic features such as drainage basins (also referred to as "watersheds"). A bioregion can be on land or at sea. The idea of bioregions was adopted and popularized in the mid-1970s by a school of philosophy called bioregionalism, which includes the concept that human culture can influence bioregional definitions due to its effect on non-cultural factors. Bioregions are part of a nested series of ecological scales, generally starting with local watersheds, growing into larger river systems, then Level III or IV ecoregions (or regional ecosystems), bioregions, then biogeographical realm, followed by the continental-scale and ultimately the biosphere.
Within the life sciences, there are numerous methods used to define the physical limits of a bioregion based on the spatial extent of mapped ecological phenomena—from species distributions and hydrological systems (i.e. Watersheds) to topographic features (e.g. landforms) and climate zones (e.g. Köppen classification). Bioregions also provide an effective framework in the field of Environmental history, which seeks to use "river systems, ecozones, or mountain ranges as the basis for understanding the place of human history within a clearly delineated environmental context". A bioregion can also have a distinct cultural identity defined, for example, by Indigenous Peoples whose historical, mythological and biocultural connections to their lands and waters shape an understanding of place and territorial extent. Within the context of bioregionalism, bioregions can be socially constructed by modern-day communities for the purposes of better understanding a place "with the aim to live in that place sustainably and respectfully."
